The extraction of fluids such as oil, gas or water from subterranean wells involves introducing a transportation pipe into the ground. The fluid is forced to the surface of the earth through the pipe by natural pressure in the well, a pump aboveground, or displacing the fluid with another fluid, such as using water to displace oil. Such a process involves a flow of highly pressurised fluid into the pipe which inevitably carries along with it debris in the form of sand, stones and other particles, which erodes the welling machinery. Therefore, it is a common practice to provide a filter assembly, known as a well screen, at the submerged opening of the transportation pipe to separate the fluid from the solids.
An available design of well screen, comprises firstly a length of perforated pipe known as a base pipe. The transportation pipe is connected at its submerged end to the base pipe. The perforations along the side of the base pipe allow the fluid to enter into the transportation pipe. Generally, it is desirable that the base pipe has as large a diameter as possible, subject to physical and efficiency constraints.
The base pipe is essentially encased in an outer layer of screen, which filters the fluid flowing into the base pipe. The layer of filter medium has fine openings, and therefore a large percentage of open area. Some types of filter medium are easily damaged as they are woven of fine metal threads, which are eroded by the particles carried by the strong fluid flow. They are also easily clogged, creating localised areas of blockage which eventually build up.
Conventionally, the filter medium is wrapped tightly around the base pipe. However, it is also proposed in the art to provide a gap between the base pipe and the filter medium, as shown in US 2002/0038707 to allow the fluid to flow past clogged areas on the filter medium and enter though unclogged adjacent areas.
Many inventions have been proposed to improve the efficiency and life spans of well screens. For example, U.S. Pat. No. 5,611,399 is concerned with fabricating a filter assembly without using welds on the filter material, since such welded seams can create areas of weakness. A base pipe with openings is disclosed, upon which is mounted a coarse screen having a series of longitudinally extending support members tied together with a wound wire which can be a series of rings. On the coarse screen is disposed a fine screen which is held by crimping. A perforated outer shroud covers the fine screen as a protective screen. The screens are put through a die in order to compress and hold these elements onto end caps.
U.S. Pat. No. 6,305,468 provides an improvement on U.S. Pat. No. 5,611,399 and is also concerned with fabricating a filter assembly without using welds on the filter material which can create areas of weakness. The method of securing the filter material is different from U.S. Pat. No. 5,611,399 and the outer shroud is also put through the die with the claimed advantage of the latter design being that the close-fit nature of the components, particularly the outer shroud and the filter material, allows the assembly to withstand significantly greater differential pressure than the constructions of prior designs such as illustrated in U.S. Pat. No. 5,611,399.
U.S. Pat. No. 6,158,507 discloses a rod-base screen with two filter layers and an outer shroud 34. The method of preparing the rod-based screen is disclosed in U.S. Pat. No. 4,314,129.
U.S. Pat. No. 4,314,129, uses a resistance welding technique to secure a spirally wound rod along a circumferential spread of longitudinal rods, the longitudinal rods running parallel along the length of the base pipe.
US Application 2002/0038707 noted above further describes a spirally-wrapped wire used to create a space between the filter and the base pipe, such that the gap between the filter medium and the base pipe is maintained. This gap prevents blockage on the surface of the base pipe in the event that the filter medium were pressed towards the base pipe.
U.S. Pat. Nos. 5,782,299 and 6,109,349 disclose a filter layer and a protective screen, which can be disposed inside or outside the filter. The protective screens are, in turn, made up of two layers of perforated stainless steel joined together. The layers are relatively thin (0.02-0.13 inch) and thus, relatively speaking, have little structural rigidity. The perforations of the two layers are mis-aligned in such a way that the fluid entering into the filter assembly cannot flow in a direct flow path, and therefore the pressure of the impingement of particles and fluid onto the fine filter mesh is reduced. The deflection is meant to reduce the direct impingement of the fluid against the filter medium.
U.S. Pat. No. 5,849,188 discloses a perforated pipe having an inner jacket closely wound on the pipe, a woven wire mesh layer and a protective jacket. The woven mesh layer is of a type known as twilled Dutch weave which, it is claimed, allows the mesh to remain relatively unclogged even when particles accumulate on the surface of the mesh.